It is available only via Steam. I know many people have problems with Steam, but basically from the splash page you are downloading the meat of the game and will have the first opportunity to play the game when it is released. I am going to guess you can make it "live" when the game goes gold, although this is only a guess.

And yes, I know this is PC discussion, but I know a lot of people are interested in this game.

I think it makes perfect sense to preload now. Can you imagine how hard the servers would get slammed on launch if they didn't allow people to preload a majority of the game before. Now we all need only a small portion of the game to complete it, which will put much less stress on the servers.